INTEGUMENT 21 



In the deeper layers there is a strongly developed network of 

 muscle-fibres showing traces of transverse striation : these are 

 inserted into the feather-sacs, and serve to erect the feathers. 



Apart from a gland present in the neighbourhood of the 

 auditory passage amongst Gallinacese, there is only a single gland 

 situated at the base of the rudimentary tail (uropygium) : this 

 uropygial gland is present in nearly all Birds, and its secretion 

 serves to oil the feathers. Dermal bones are characteristically 

 absent, while epidermic structures, such as feathers, claws, spurs, 

 foot-scales, and beak-sheaths, are strongly developed. 



One of the most marked characteristics of Birds is the pos- 

 session of feathers. In the majority of Birds they are of two 

 kinds down-feathers and contour-feathers, and are usually 

 arranged in so-called feather-tracts (pterylce) separated by naked 

 regions (apteria). The base of each feather is embedded in 

 an epidermic sac or follicle. Their mode of development corre- 

 sponds essentially with that of the epidermic scales of Keptiles. 



In the region where a feather is to be formed, the dermal tissue becomes 

 raised up towards the ectoderm (Fig. 14, A), and thus gives rise to a vas- 

 cular papilla. As this papilla grows out to form an elongated cone with a 

 pointed apex, the feather-germ (B), its base sinks gradually deeper and deeper 

 into the dermis, and thus becomes surrounded by a sort of pocket the 

 feather-follicle. The horny, as well as the Malpighian layer of the epidermis 

 extends into the base of the follicle, and thence into the feather-germ, the 

 interior of which is throughout filled by cells of the dermis, which give rise to 

 the pulp. As the feather-germ keeps 011 growing, the cells of the Malpighian 

 layer begin to proliferate rapidly, giving rise to a series of radial folds 

 arranged along a central axis, which extend inward towards the pulp, and 

 are immediately bounded by the horny layer (C). These folds, between 

 which the nutritive pulp extends, then become cornified and separated from 

 above downwards from the surrounding cells ; and, by a gradual drying of 

 the central pulp-substance, give rise to a tuft of horny rays, which are, 

 however, at first bound together by the enclosing stratum corneum. Most 

 Birds are hatched when the feathers are in this stage of development, and 

 they thus appear as if covered with brush-like hairs. 



By the shedding of the surrounding horny layer the rays or barbs become 

 free (D), and if these are all similar to one another, an embryonic down- 

 feather is formed. The whole feather-germ, however, does not become 

 divided up into barbs in this manner : its lower portion, embedded in the 

 skin, retains a more uniform character and forms the quill (calamus). 



The embryonic down-feathers (E), on the individual barbs of which 

 smaller secondary rays or barbules become developed, may retain their char- 

 acter as such throughout life or may be replaced by definitive feathers. In 

 this case a second, larger, follicle early arises from the base of the follicle of 

 the down-feather, the pulp of the two being in connection (D). The papilla 

 developing within the interior of this new follicle grows rapidly, gradually 

 pushes the base of the down-feather out of its follicle, and comes to the 

 surface. 



Each contour feather (penna) at first closely resembles a down- 

 feather (pluma) in structure, and consists of a tuft of similar rays 

 or barbs provided with secondary rays or barbules. In the course 

 of further growth, however, one of the rays becomes rapidly 



